A non-volatile memory is a semiconductor device in which data is rewritable and stored data is retained even when the power is turned off. Flash memory is one type of non-volatile memory that has become increasingly popular. In a flash memory, a transistor which constitutes a memory cell has a “charge storage layer” of either a floating gate or an insulating film. Data is stored in a flash memory by accumulating electrical charges in this charge storage layer.
In recent years, various methods to increase the amount of data that can be stored in a single memory cell have been proposed. For example, there is a virtual ground flash memory in which two charge storage regions are provided in a charge storage layer of a single memory cell by switching around a source region and a drain region in operation. With this technique, two bits of data can be stored in a single memory cell instead of one.
Other methods have been proposed to expand the data storage ability. Japanese Patent Application Publication No. JP-A-2006-24922, for example, describes another conventional solution to provide two charge storage regions in a charge storage layer by providing two electrodes on the charge storage layer in a memory cell to store two bits of data in a single memory cell. Furthermore, Japanese Patent Application Publication No. JP-A-2004-343014, for example, describes a conventional technique of a flash memory having a charge storage layer separated from under both ends of a gate electrode to its side surfaces.
However, these solutions are not without their flaws. In a virtual ground flash memory with a conductive film as a charge storage layer, for example, it is required to separate the charge storage layer in a channel direction in a memory cell since stored electric charges move around in the charge storage layer. Meanwhile, in a virtual ground flash memory with an insulating film as a charge storage layer, an influence of a so-called complementary bit disturb (CBD) in which electrical charges stored in two charge storage regions interfere with each other becomes large unless the charge storage layer is separated in a channel direction in a memory cell. As a consequence, separation of electrical charges stored in two charge storage regions becomes difficult. Furthermore, if the charge storage layer is connected between adjacently arranged memory cells, threshold voltages of the adjacently arranged memory cells may be influenced as the electrical charges stored in the charge storage layer move around in the charge storage layer.